1. Field of the Invention
The subject invention relates to latent catalysts for heat curable thermosetting resins. More particularly, the subject invention relates to a new class of catalysts which have improved latency at ambient temperatures and which are oxidation and hydrolysis resistant. These catalysts are useful in catalyzing a variety of resin systems, particularly epoxy and bismaleimide resins.
2. Description of the Related Art
Latent catalysts are necessary for the cure of several important categories of resins. The cure of epoxy resins, for example, is generally catalyzed even when "curing agents" such as acid anhydrides or organic diamines are present. Bismaleimide resins may be cured by heat along, but the properties of the cured products are not as good as those achieved when the same resin system is cured catalytically.
Latent catalysts may be divided into two principle groups: those which are activated photochemically, and those which are activated thermally. Photo-activated catalysts are extensively used in the electronics industry in formulating resins useful in processing integrated circuits and circuit boards. In the structural materials area, however, where part sizes are generally much larger and often of complex shape, thermally-activated catalysts are the norm. The discussion which follows relates to this type of latent catalyst.
An ideal latent catalyst will have little or no catalytic activity below a certain threshold temperature, above which the catalyst becomes active. This threshold temperature must be high enough above ambient to facilitate long term storage of the uncured, catalyzed resins and products containing them, but must be low enough to be convenient for the cure of the particular resin system with which the catalyst is used.
When the catalyst has a latency threshold which is too high, several deleterious effects may occur, including thermal decomposition of the resin components; volatilization of low molecular weight resin components or solvents; and extensive thermal curing in conjunction with catalytic cure of the resin. Additionally, subjecting assemblies such as structural composites to wide temperature ranges during processing may cause distortion due to uneven expansion and contraction.
In U.S. Pat. No. 3,562,215, for example, are disclosed substituted urea and guanidine compounds which are used in conjunction with a glycol and with organic lead or mercury derivatives to form a latent catalyst system for epoxy resins. However the catalyst must be used in amounts of approximately ten percent by weight relative to the epoxy resins. Furthermore it is well known that the decomposition of ureas can produce a number of volatile products. Finally, organolead and organomercury compounds are highly poisonous, difficult to handle safely, and may cause environmental problems.
The use of phosphonium halides in conjunction with alkali metal hydroxides or halides are disclosed as latent catalysts for use in epoxy resins in U.S. Pat. No. 4,320,222. However the presence of alkali metal salts may be deleterious to the long term stability of the cured resin. Moreover, the decomposition of the phosphonium halide involves the formation of an organic halide which, in most cases, is relatively volatile. Finally, the phosphonium halides are water soluble and possess strong biocidal properties, thus presenting safety and environmental concerns.
The use of organophosphines and organophosponium salt as latent catalysts for bismaleimide resin systems is disclosed in U.S. Pat. No. 4,644,039. However, organophosphines, particularly the aliphatic phosphines, are volatile compounds of high toxicity and thus also present handling problems. Moreover, many organophosphines, again particularly the aliphatic organophosphines, are readily oxidizable to phosphine oxides which possess little or no catalytic activity. Thus formulation of systems containing these catalysts preferably takes place in inert atmospheres, and subsequent oxidation in situ is possible, causing gradual loss of catalytic activity.
In U.S. Pat. No. 4,131,633 are disclosed latent catalysts prepared by the reaction of maleic anhydride and tris-substituted phosphines. However, the latency threshold of these compounds is rather low, and the products are additionally highly water sensitive, being subject to complete hydrolysis.
It would be desirable to prepare a latent catalyst which is oxidatively and hydrolytically stable under ordinary conditions; which presents a minimum of handling problems; which has a latency threshold which allows for long term storage of uncured products; which is active at a temperature suitable for use with advanced resin systems; and which produces low levels of volatiles upon cure.